Time and the River Flowing

One of the most difficult concepts for people to comprehend
when they encounter geology is the immense time frame that geologists
use. To a geologist a thousand years is an instant. Most geologic
changes and events take place over millions of years. The exception
would be the rare cataclysmic event such as a meteorite impact.
Through their training, geologists learn to think in terms of millions
of years. The age of the earth is approximately 4.6 billion years,
a number that is essentially incomprehensible to the vast majority
of people.

Through the eighteenth and nineteenth centuries geologists
became aware of the necessity for very long periods of time to explain
the vast change that their research was finding. Geologists developed
a Relative Time Scale and tried to estimate
through various means the actual lengths of time and the age of
the earth. This relative time scale is based on the study of rock
layers (stratigraphy) and the study of fossils (paleontology)
and is still in use today. The strange sounding names: Ordovician,
Cretaceous, Quaternary, etc., are still used by geologists
and refer not just to rocks and fossils but to periods of geologic
history.

The discovery of radioactivity in the 1890s was quickly
put to use by geologists to create an absolute time scale. It was
important to be able to tell the age of a rock layer in terms of
millions of years rather than knowing that this rock is older than
that rock. Radioactive elements within the rocks themselves provided
a built-in clock. This natural clock works because the rate of radioactive
decay or change is a constant. Geologists quickly learned to read
these clocks and put absolute ages on the periods of the Geologic
Time Scale.